US 20050147167 A1
An effective method for dynamically selecting the number of I, P and B frames during video coding is proposed. Short-term look-ahead analysis of a video sequence yields a variable number of B frames to be coded between any two stored pictures. The first picture of a group of frames (GOF) may be coded as a B picture. Motion speed is calculated for each picture of the GOF with respect to the first picture of the GOF. Subject to exceptions, as long as the subsequent pictures exhibit motion speeds that are similar and motion vector displacements that are co-linear with those of the first picture in the GOF, they may be coded as B pictures. When a picture is encountered having a motion speed that is not the same as that of the first picture in the GOF, the picture may be coded as a P picture. In some embodiments, a sequence of B pictures that terminates in a P picture may be called a “group of frames” (GOF).
1. A method of assigning frame types for coding of pictures in a video sequence, comprising:
determining movement between image data of a reference picture and each of a plurality of pictures in temporal order,
comparing the movement of a first picture in the plurality, temporally closest to the reference picture, to movement of other pictures therein,
if the plurality of pictures exhibits consistent motion speed, assigning the pictures therein as B pictures.
2. The method of
3. The method of
determining whether a scene change occurs in the plurality of pictures, and, if so:
coding a first picture temporally after the scene change as a P picture, and
coding all pictures in the plurality that occur from the first picture to a picture immediately prior to the scene change as B picture long as they exhibit consistent motion speed.
4. The method of
5. The method of
6. The method of
7. The method of
8. The method of
9. The method of
10. A video coding method, comprising, from a sequence of video data:
determining a motion speed between a first picture and a reference picture;
for each picture following the first picture, until a termination condition is met:
determining a motion speed for the respective picture,
comparing the motion speed of a respective picture with the motion speed of the first picture, and
coding the respective picture as a B picture if the motion speeds are consistent with each other; and
when the termination is met, coding a picture as a P picture.
11. The video coding method of
12. The video coding method of
13. The video coding method of
14. The video coding method of
15. The video coding method of
16. The video coding method of
n and n+1 identify pictures between which the scene change may be detected, xn(i,j) and xn+1(i,j) respectively represent pixel values of pictures n and n+1 at pixel locations (i,j) and M and N represent width and height (respectively) of pictures n and n+1.
17. The video coding method of
wherein the termination condition is met when a random access policy dictates that a picture be coded as an I picture, and
a picture temporally adjacent to and before the I picture is coded as a P picture.
18. Apparatus, comprising:
a memory to store pictures of a video sequence,
a video coder coupled to the memory, to code each stored picture as one of an I picture, a P picture and a B picture,
a motion vector generator coupled to the memory to generate motion vectors for a plurality of stored pictures, the motion vectors measuring motion between a respective picture and a stored reference picture,
a colinearity detector having an input coupled to the output of the motion vector generator, and
a picture type decision unit to assign pictures having generally consistent motion speeds for B picture coding and to assign a picture that does not exhibit consistent motion speed for coding as a P picture.
19. The apparatus of
20. The apparatus of
21. The apparatus of
22. The apparatus of
23. The apparatus of
24. The apparatus of
25. A coded video signal created from a method comprising:
from a sequence of video data, determining a motion speed between a first picture and a reference picture;
for each picture following the first picture in the sequence, until a termination condition is met:
determining a motion speed for the respective picture,
coding the respective picture as a B picture if its motion speed is consistent with the motion speed of the first picture;
when the termination is met, coding a picture as a P picture; and
outputting coded data of the B pictures and the P picture to a channel.
26. The coded video signal of
27. The coded video signal of
28. The coded video signal of
wherein the termination condition is met when a random access policy dictates that a picture be coded as an I picture, and
a picture temporally adjacent to and before the I picture is coded as a P picture.
29. A video coding assignment method, comprising, for each of a plurality of pictures following a reference picture:
adding a first picture that follows the reference picture in display order to a group of frames,
determining a motion speed of the first picture with respect to the reference picture,
iteratively, for pictures subsequent to the first picture in display order:
adding the subsequent picture to the group of frames,
determining a motion speed of the subsequent picture with respect to the reference picture,
if the motion speed of the subsequent picture is consistent with the motion speed of the first picture, performing a next iteration, and
if not, coding the last picture of the group of frames as a P picture and coding all other pictures in the group of frames as a B picture.
30. The video coding assignment method of
31. The video coding assignment method of
S(n,b) represents the motion speed of a picture, dx and dy represent displacements of the respective pixelblock b of picture and n represents the temporal distance of the picture from the reference picture.
32. The video coding assignment method of
e(n,b) represents a difference of motion vector displacements of the respective subsequent picture with respect to the first picture, each scaled according to its temporal distance from the reference picture, and Nblocks represents the number of pixelblocks in each picture.
Video encoder optimization for bit rate reduction of the compressed bitstreams and high visual quality preservation of the decoded video sequences encompasses solutions such as rate-distortion optimized mode decisions and parameter selections, frame type selections, background modeling, quantization modeling, perceptual modeling, analysis-based encoder control and rate control.
Generally, many video coding algorithms first partition each frame or video object plane (herein, “picture”) into small subsets of pixels, called “pixelblocks” herein. Then each pixelblock is coded using some form of predictive coding method such as motion compensation. Some video coding standards, e.g., ISO MPEG or ITU H.264, use different types of predicted pixelblocks in their coding. In one scenario, a pixelblock may be one of three types: Intra (I) pixelblock that uses no information from other pictures in its coding, Unidirectionally Predicted (P) pixelblock that uses information from one preceding picture, and Bidirectionally Predicted (B) pixelblock that uses information from one preceding picture and one future picture.
Consider the case where all pixelblocks within a given picture are coded according to the same type. Thus, the sequence of pictures to be coded might be represented as
Since B pictures use information from future pictures, the transmission order is usually different than the display order. For the above sequence, transmission order might occur as follows:
Thus, when it comes time to decode B2 for example, the decoder will have already received and stored the information in I1 and P5 necessary to decode B2, similarly B3 and B4.
The receiver then reorders the sequence for proper display. In this operation I and P pictures are often referred to as “stored pictures.”
The coding of the P pictures typically utilizes Motion Compensation predictive coding, wherein a Motion Vector is computed for each pixelblock in the picture. Using the motion vector, a prediction pixelblock can be formed by translation of pixels in the aforementioned previous picture. The difference between the actual pixelblock in the P picture and the prediction block, (the residual) is then coded for transmission.
Each motion vector may also be transmitted via predictive coding. That is, a prediction is formed using nearby motion vectors that have already been sent, and then the difference between the actual motion vector and the prediction is coded for transmission. Each B pixelblock typically uses two motion vectors, one for the aforementioned previous picture and one for the future picture. From these motion vectors, two prediction pixelblocks are computed, which are then averaged together to form the final prediction. As above the difference between the actual pixelblock in the B picture and the prediction block is then coded for transmission.
As with P pixelblocks, each motion vector of a B pixelblock may be transmitted via predictive coding. That is, a prediction is formed using nearby motion vectors that have already been transmitted, and then the difference between the actual motion vector and the prediction is coded for transmission.
However, with B pixelblocks the opportunity exists for interpolating the motion vectors from those in the co-located or nearby pixelblocks of the stored pictures. The interpolated value may then be used as a prediction and the difference between the actual motion vector and the prediction coded for transmission. Such interpolation is carried out both at the coder and decoder.
In some cases, the interpolated motion vector is good enough to be used without any correction, in which case no motion vector data need be sent. This is referred to as Direct Mode in H.263 and H.264. This works particularly well when the camera is slowly panning across a stationary background. In fact, the interpolation may be good enough to be used as is, which means that no differential information need be transmitted for these B pixelblock motion vectors. Within each picture the pixelblocks may also be coded in many ways. For example, a pixelblock may be divided into smaller sub-blocks, with motion vectors computed and transmitted for each sub-block. The shape of the sub-blocks may vary and need not be square.
Within a P or B picture, some pixelblocks may be better coded without using motion compensation, i.e., they would be coded as Intra (I) pixelblocks. Within a B picture, some pixelblocks may be better coded using unidirectional motion compensation, i.e., they would be coded as forward predicted or backward predicted depending on whether a previous picture or a future picture is used in the prediction.
Prior to transmission, the prediction error of a pixelblock or sub-block is typically transformed by an orthogonal transform such as the Discrete Cosine Transform or an approximation thereto. The result of the transform operation is a set of transform coefficients equal in number to the number of pixels in the pixelblock or sub-block being transformed. At the receiver/decoder, the received transform coefficients are inverse transformed to recover the prediction error values to be used further in the decoding.
Not all the transform coefficients need be transmitted for acceptable video quality. Depending on the transmission bit rate available more than half, sometimes much more than half, of the transform coefficients may be deleted and not transmitted. At the decoder their values are replaced by zeros prior to inverse transform.
Also, prior to transmission the transform coefficients are typically quantized and entropy coded. Quantization involves representation of the transform coefficient values by a finite subset of possible values, which reduces the accuracy of transmission and often forces small values to zero, further reducing the number of coefficients that are sent. In quantization typically, each transform coefficient is divided by a quantizer step size Q and rounded to the nearest integer. For example, the transform coefficient C would be quantized to the value Cq according to:
The sub-block size and shape used for motion compensation may not be the same as the sub-block size and shape used for the transform. For example, 16×16, 16×8, 8×16 pixels or smaller sizes are commonly used for motion compensation whereas 8×8 or 4×4 pixels are commonly used for transforms. Indeed the motion compensation and transform sub-block sizes and shapes may vary from pixelblock to pixelblock.
A video encoder must decide what is the best way amongst all of the possible methods (or modes) to code each pixelblock. This is known as the mode selection problem. Depending on the pixelblock size and shape, there exist several modes for intra and inter cases, respectively.
A video encoder must also decide how many B pictures, if any, are to be coded between each I or P picture. This is known as the frame type selection problem, and again, ad hoc solutions have been used. Typically, if the motion in the scene is very irregular or if there are frequent scene changes, then very few, if any, B pictures should be coded. On the other hand, if there are long periods of slow motion or camera pans, then coding many B-pictures will result in a significantly lower overall bit rate. Moreover, a higher number of coded B frames makes possible achieving temporal/computational scalability at the decoder without impacting greatly the visual quality of the decoded sequence and the computational complexity of the decoder. Consequently, platforms and systems with various CPU and memory capabilities can make use of streams coded using numerous B frames.
Modern encoders typically select the number of B frames that occur between each I or P picture to be equal to one or two. This predetermined and somewhat arbitrary decision is motivated by experimental work, which shows that for most video sequences the above decision reduces the bit rate without affecting negatively the visual quality of the decoded sequences. The opportunity exists, however, to reduce the bit rate much more for sequences that exhibit slow motion or camera pans by increasing the number of B frames. It is believed that current coding systems do not take advantage of this opportunity, due to (a) the difficulty of the I/P/B decision and (b) the increase in the encoder's computational complexity that the implementation of the frame type decision would determine. Indeed, the appropriate number of B frames to be coded for each sequence not only depends on both the temporal and spatial characteristics of the sequence but it may vary across the sequence as the motion characteristics often change and a selection of different numbers of B frames for each different part of the sequence is typically required. Accordingly, there is a need in the art for a computationally inexpensive coding assignment scheme that dynamically assigns a number of B pictures to occur between reference pictures (I- and P- pictures) based on picture content.
Embodiments of the present invention provide a frame type selector for a video coder. This selector assigns input pictures from a video sequence for intra coding, predictive coding or bidirectionally predictive coding. According to the embodiment, the first picture following an I or P picture may be coded as a B picture. For all pictures subsequent thereto, motion speed may be calculated with respect to the reference picture, the I or P picture. Subject to exceptions, as long as the subsequent pictures exhibit generally similar, constant or almost constant motion speed, they may be coded as B pictures. When a picture having an irregular motion speed is encountered, then that picture may be coded as a P picture. In some embodiments, a sequence of B pictures that terminates in a P picture may be called a “group of frames” (GOF). The frame with irregular motion speed may terminate a current GOF.
The motion vector generator 120, as its name implies, identifies relative motion between image information in the current picture n and the reference picture of the previous GOF. Motion vector calculation is well known in the video coding arts. Generally, it involves comparing blocks of image data from a candidate picture of video data (picture n) to blocks of image data in the reference picture that are generally spatially co-incident. If a matching block is found in the reference picture, the motion vectors represent spatial displacement between the block's location in picture n and the matching block's location in the reference picture. Thus, a set of motion Vectors is generated for each pixelblock in picture n. The motion vector generator 120 may output motion vectors (labeled, “MV” in
The colinearity detector 140 determines whether the motion vectors of the new picture n demonstrate a general flow of motion that is consistent with the flow of motion obtained from a prior sequence of pictures (from the prior reference picture P to picture n−1). The colinearity detector 140 may generate an output representing a degree of difference between the colinearity of motion vectors of picture n and the motion vectors of the first picture in the GOF of the video sequence.
The scene change detector 130, as its name implies, can identify scene changes in the source video data. Various scene change detectors 130 are known in the art and can be integrated into the system of
The picture type decision unit 150 may determine how each picture is to be coded. It generates control signals to the picture buffer 160 and video coder 170 in response to these picture assignment decisions. When the picture type decision unit 150 assigns the current picture n to be coded as a B-picture, it may cause the video data of picture n and its associated motion vectors to be stored in the buffer pool 160 to await later coding and may advance operation to the next picture (picture n+1).
When the picture decision unit 150 determines that picture n shall be coded as a P picture, the picture decision unit 150 may enable the video coder 170, causing it to code all pictures of the GOF stored in the buffer pool 160. All pictures that follow the previously coded P picture, including the newly assigned P picture and any B pictures that occur between the new P picture and the previously coded P picture are coded by the video coder 170. Operation of the frame type selector 150 may advance to a new input picture n+1 and repeat the above decision steps using the frames of the new GOF.
In an embodiment, the picture decision unit 150 also could decide to code a picture as an I picture to satisfy other coding policies that are provided to support random access to video frames and the like. In this case, the picture type decision unit 150 may also cause the video coder 170 to code all pictures resident in buffer pool 160 up to and including the newly assigned I picture.
As the foregoing description indicates, the frame type selector 100 may process groups of frames from input video data. Each GOF may have the form BB . . . BP (or, alternatively, BB . . . BI). When input image data indicates generally consistent (i.e., similar, constant or almost-constant speed) motion among video content, the pictures that exhibit the consistent motion are assigned as B pictures to the extent possible. When the constant motion speed terminates, a picture may be designated as a P picture. The B pictures may be coded using the P picture of the previous group of pictures and the newly identified P picture as reference pictures. Because all of the B pictures are identified as exhibiting generally constant motion speed, coding should be particularly efficient.
Video coders 170 typically include a decoding chain that reconstructs image data in a manner that replicates operations to be performed by a decoder (not shown) that receives coded video data from a channel. Here, the decoding chain is shown as including a decoder 230, a reference picture store 240, and a motion or spatial predictor 250. The decoder 230 inverts operation of elements 180-210 and generates reconstructed image data that can be stored 240 as reference pictures for further prediction. Reference pictures in storage 240 also may be input to the motion vector generator 120 for use in building GOFs as described above. For motion prediction in P and B frames, the motion predictor 250 may forward selected image data from the reference pictures motion vectors to the subtractor 180. For motion prediction in P or B coding modes, the selected image data is identified by the motion vectors, which In embodiments of the present invention, some of the motion vectors can be generated by the motion vector generator 120.
According to an embodiment, the picture type assignment techniques illustrated in
In another embodiment, when a scene cut occurs between two pictures n and n−1 and the picture before the scene cut n−1 is not the first frame of GOF, then a picture type decision may assign picture n−1 as a P frame and picture n as either an I or a P frame. In this embodiment, the pictures n−1 and n may be coded at either full quality of low quality. Full quality means using the same coding parameters as for previous pictures. Low quality means reducing the spatial quality of the picture, typically by increasing the value of the quantization in 200.
In a further embodiment, when a scene cut occurs between two pictures n and n−1, picture type decision may permit the GOF to continue and assign a B frame to the next picture after the scene cut (picture n). When the maximum number of B frames (decided in the coding system) has been exceeded, a new frame may be assigned for P coding, yielding a pattern PB . . . B||B . . . BP (where || represents the position of the scene cut). Optionally, B pictures that are members of a GOF that includes a scene cut may be coded at low quality relative to pictures from other GOFs.
The picture type decision scheme discussed so far provides several advantages in video coding applications. First, because it favors coding of consecutive pictures that exhibit similar motion properties (i.e., constant or almost-constant motion speed) as B pictures, it yields lower bit rates of the compressed streams. Second, the picture type decision scheme is computationally inexpensive. The computation of motion speeds and speed errors requires simple operations. Moreover, the motion vectors computed for the purpose of frame type decision are re-used during the coding of B and P pictures. Thus, in the aggregate, the expense associated with the picture type assignment scheme of the present embodiments is minimal. Third, coding using several B pictures in appropriate contexts also provides a simple form of scalability for use with video decoders of varying capability. B pictures typically are not reference pictures for other pictures and, therefore, some video decoders can elect to drop selected B pictures to simplify their decoding operation and still obtain useful reconstructed data.
The picture type assignment scheme of the foregoing embodiments provides advantages over, for example, a brute force approach that simply would code every combination of B pictures and pick the combination that minimized bit rate of the output coded video signal. The brute force approach is far too complex. It would requires a very large number of trial-and-error operations, most of which must be discarded once a final decision is made. By contrast, the present invention provides a frame type assignment scheme that requires far less computational expense and higher efficiency—as noted, motion vector computations from frame type assignment may be re-used when the video data is coded.
For pictures other than the first picture in the video sequence, the method 1000 may determine whether a scene cut has occurred. In one embodiment, the method 1000 computes a correlation coefficient between the current picture n and the previous picture n−1 (box 1050). If the correlation coefficient is higher than some predetermined threshold (box 1060), then the method 1000 may determine that no scene cut occurred (box 1070). Thereafter, the method may determine whether the nth picture causes a length of a current group of pictures to meet a predetermined maximum length set for the system (box 1080). If so, then picture n may be assigned to be a P-picture (box 1090). The P-picture decision terminates the current GOF (box 1100) and causes the video pictures of the GOF to be coded (box 1110). Thereafter, unless the method 1000 has reached the end of the video sequence (box 1120), the method advances to the next picture (box 1040) and repeats operation (box 1010).
If at box 1080 the method 1000 determines that the nth picture does not cause the maximum GOF length to be reached, the method may compute forward motion vectors between picture n and the reference picture of the previous GOF (typically, a P picture) (box 1130) and also compute the slope of the motion vector displacements (box 1140). If the current picture n is the first picture of a new GOF (box 1150), the method may assign the picture's type to be a B-picture (box 1160) and advance operation to the next picture (boxes 1040, 1010). Otherwise, the method 1000 may compute a speed error from the displacement slopes of the current picture and the first picture in the GOF (box 1170). If the speed error exceeds some predetermined threshold (box 1180), then the picture may be assigned as a P-picture (box 1090). Again, the P picture assignment terminates a current GOF and causes pictures of the GOF to be coded (boxes 1100, 1110).
If the speed error does not exceed the threshold (box 1180), the method 1000 may determine whether the current picture is the last picture of the video sequence (box 1190). If so, the method 1000 again may advance to box 1090, assign the picture to be a P-picture and code the GOF (boxes 1100, 1110) before terminating. Otherwise, the method 1000 may assign the current picture to be a B-picture (box 1200) and advance operation to the next picture in the video sequence (boxes 1040, 1010).
Returning to box 1060, if the correlation coefficient is smaller than the scene cut threshold, the method 1000 may determine that a scene cut occurred (box 1210). The method may assign a picture type based on a scene management policy for the system (box 1220). In the simplest embodiment, the scene management policy may dictate that the first picture following a scene cut shall be coded as an I-picture. Other embodiments may assign to code the picture as either an I-picture or P-picture depending upon the relative bandwidth consumed by these coding choices. If the picture is assigned to be an I-picture or a P-picture, the assignment terminates the GOF (box 1100) and causes pictures therein to be coded (box 1110).
Further, other embodiments may assign to code the picture after the scene cut as the picture type decision dictates, with the provision that, in the case such a decision is to encode the picture as a B frame, measures are taken to prevent the B frame from referencing any picture prior to the scene cut.
In one embodiment, a scene cut decision may be made based upon a correlation coefficient established for each of two temporally adjacent frames. A correlation coefficient C for a frame n may be computed according to:
In practice, of course, perfect colinearity will not always be observed. Accordingly, the motion vector of the first picture in a GOF (picture 1 in the example of
According to an embodiment, motion vectors may be determined for all pixelblocks in a candidate picture. Again, let dx and dy by the components of a motion vector (displacements) along the x and y directions. If a scene cut does not exist between a first picture of a GOF and the preceding picture, it can be assumed that the first picture of the GOF is a B-picture (picture no. 1). Starting with the first picture (picture 1), for each picture of the GOF, the system may compute the motion speed. The motion speed of a block b in the picture may be measured by slopes Sx(n, b) and Sy(n, b) and S(n, b) as follows:
Thus, an error value can be obtained for each image block in the candidate picture. The system may compute the speed error for picture n (i.e., E(n)) as the mean of absolute speed errors of all blocks in the picture, in which case E(n) is given by:
The foregoing picture type decision scheme contributes to highly efficient coding of pictures. At a high level, the picture assignment scheme identifies pictures that exhibit a common motion speed and small speed errors among them. When these characteristics are identified, the picture type decision scheme classifies a relatively large number of candidate pictures as B pictures.
Tying frame type decisions to an observable pattern of motion speeds among pictures also can yield additional advantages in terms of coding effectiveness. Pictures may be coded according to the direct mode in H.263+ and H.264. In other words, not only more B frames are encoded, but they are also coded effectively. As illustrated in
Several embodiments of the present invention are specifically illustrated and described herein. However, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.